Automotive heating, ventilation, and air conditioning (HVAC) systems generally incorporate a large, hollow housing behind the instrument panel, which contains a cold evaporator core and hot heater core, as well as various valves for directing air flow through them and ultimately out into the passenger compartment.
During air conditioning operation, refrigerant is circulated through the evaporator core to cool and dehumidify air before it is directed to the passenger compartment. Moisture condenses within the air passages of the evaporator core and in other portions of the HVAC system. This collected moisture, in some instances, promotes the growth of microorganisms. If the growth is excessive, it may produce an unpleasant odor or otherwise deleteriously affect passenger comfort.